The transport of specific molecules across lipid membranes is an essential function of all living organisms and a large number of specific transporters have evolved to carry out this function.
One subfamily of transporters is the amino acid transporters. One member of the amino acid transporters is solute carrier family 19 (thiamine transporter), member 2 (SLC19A2). SLC19A2 is also referred to as reduced folate carrier protein (RFC) like, and thiamine transporter (THTR1). The murine SLC19A2 gene is also referred to as DDA1 and AW322295.
Thiamine-responsive megaloblastic anemia syndrome (TRMA), also known as Rogers syndrome, is an early-onset, autosomal recessive disorder defined by the occurrence of megaloblastic anemia, diabetes mellitus, and sensorineural deafness, responding in varying degrees to thiamine treatment. Neufeld et al., Am. J. Hum. Genet. 61: 1335–1341 (1997) and Raz et al., Hum. Mutat. 16: 37–43 (1998) narrowed the TRMA locus from a 16- to a 4-cM interval on 1q23.3, and Banikazemi et al., Molec. Genet. Metab. 66: 193–198 (1999) further refined the locus to a 1.4-cM interval. Studies by Rindi et al., J. Inherit. Metab. Dis. 17: 667–677 (1994) and by Stagg et al., J. Clin. Invest. 103: 723–729 (1999) had suggested that deficiency in a high-affinity thiamine transporter may cause this disorder.
Labay et al., Nature Genet. 22: 300–304 (1999) identified the SLC19A2 gene by positional cloning. They assembled a P1-derived artificial chromosome (PAC) contig spanning the TRMA candidate region. This clarified the order of genetic markers across the TRMA locus, provided nine new polymorphic markers, and narrowed the locus to an approximately 400-kb region. Labay et al. (1999) found that the SLC19A2 gene consists of six exons spanning approximately 22.5 kb.
Due to its homology with SLC19A1, a reduced folate carrier protein, Diaz et al., Nature Genet. 22: 309–312 (1999) identified the SLC19A2 gene in the critical region 1q23.2-q23.3 and cloned the entire SLC19A2 coding region by screening a human fetal brain cDNA library. The SLC19A2 gene encodes a protein of 497 amino acids predicted to have twelve transmembrane domains. Northern blot analysis detected a 4-kb transcript in all tissues tested, most abundantly in skeletal and cardiac muscle.
Fleming et al., Nature Genet. 22: 305–308 (1999) used a candidate gene approach to identify putative thiamine transporters in the 1q23.3 critical region and found mutations in the SLC19A2 gene in two families with TRMA. Fleming et al. (1999) demonstrated that the SLC19A2 gene encodes a functional thiamine transporter.
In all affected individuals in six TRMA families, Labay et al. (1999) found mutations in the SLC19A2 gene. They suggested that a defect in the thiamine transporter protein encoded by this gene, called THTR1 by them, may underlie the TRMA syndrome. Among four Iranian families with TRMA, Diaz et al. (1999) identified two frameshift mutations in exon 2, a 1-bp insertion and a 2-bp deletion, of the SLC19A2 gene. Raz et al., Hum. Mutat. 16: 37–43 (2000) summarized knowledge on mutations in the SLC19A2 gene in TRMA patients and identified four novel mutations.
Lo et al., J. Biol. Chem. 276 (40), 37186–37193 (2001), reported identifying a p53-inducible gene by performing mRNA differential display on IW32 murine erythroleukemia cells containing a temperature-sensitive p53 mutant allele, tsp53(Val-135). Sequence analysis of the full-length cDNA revealed its identity as the mouse homologue of the human thiamine transporter 1 (THTR-1). Induction of the mouse THTR-1 (mTHTR-1) mRNA was detectable as early as 1 hour at 32.5 degrees C.; upon shifting back to 38.5 degrees C., mTHTR-1 transcript was rapidly degraded with a half-life of less than 2 hours. Elevation of mTHTR-1 expression was found in DNA damage-induced normal mouse embryonic fibroblast cells, but not in p53(−/−) mouse embryonic fibroblast cells, suggesting that mTHTR-1 induction was p53-dependent. A region within the first intron of the mTHTR-1 gene bound to p53 and conferred the p53-mediated transactivation. Furthermore, increased thiamine transporter activities were found in cells overexpressing mTHTR-1 and under conditions of DNA damage or p53 activation. The authors concluded that p53 may be involved in maintaining thiamine homeostasis through transactivation of THTR-1.
Lo and Wang, Biochim. Biophys. Acta 1576 (1–2), 209–213 (2002), reported that the mouse THTR-1 gene is predicted to encode a protein of twelve hydrophobic stretches and a hydrophilic loop of 87 amino acids between transmembrane helices VI and VII. They also reported the cloning of mouse THTR-1 a gene, with identification of two major transcriptional start sites located at −175 and −183 relative to the translation start codon. In addition, the authors reported cloning a spliced variant, designated THTR-1b, from mouse liver cDNA library. This isoform is characterized by an inframe deletion of 114 nucleotides from the 3′-terminal region of exon 2, predicting the expression of a truncated protein lacking the central 38 amino acids of the loop region. THTR-1b coexpressed with THTR-1a in many of the mouse tissues and in day-7 to day-17 embryos, but in lower levels than the THTR-1a. When expressed in mammalian cells, both isoforms were able to mediate the transport of thiamine. Therefore, the authors concluded that the transport function of the mouse THTR-1 is not determined by the central 38 amino acids of its loop region.
The complete 3554 bp nucleotide sequence for the murine SLC19A2 gene has been deposited in GenBank (Accession: AF179403; GI: 12002903).
Given the importance of transporters, particularly amino acid transporters such as SLC19A2, a clear need exists for the elucidation of their functions, which information can be used in preventing, ameliorating or correcting dysfunctions or diseases associated therewith.